Scientists
have been working on ways for decades to trap tiny microscopic parts of a cell.
Today, scientists are going even further with the ability to trap even smaller
nanoscale particles using optical tweezers. The original optical tweezers used
on microscopic particles had issues with overheating that made them impractical
for use with smaller nanoscale particles.

Engineers at Harvard have been able to create a device that makes it easier to
isolate nanoparticles like viruses and tiny individual cellular components. The
optical tweezers use light from a laser to trap particles. The light is shined
through a microscope and the new design prevents overheating that was common in
older designs.

"We can get beyond the limitations of conventional optical tweezers,
exerting a larger force on a nanoparticle for the same laser power," says
principal investigator Ken Crozier, Associate Professor of Electrical
Engineering at the Harvard School of Engineering and Applied Sciences (SEAS).

The next generation optical tweezers are being called plasmonic nanotweezer and have an
integrated heatsink. The smaller the particles scientists are trying to trap,
the more laser energy is needed. The problem was that the researchers found
that the higher power laser would cause the water that the particles they
wanted to trap were suspended in to boil. The boiling occurred even when the
team used a heatsink made up of tiny gold discs submerged into the water.

The team found that replacing the sheet of glass with gold discs on it with a
sheet of silicon coated in copper and gold with raised gold pillars they were
able to dissipate the heat from the laser. That means the water didn’t boil and
the particles the scientists wanted to trap were reachable.

"The gold, copper, and silicon under the pillars act just like the heat
sink attached to the chip in your PC, drawing the heat away," says lead author
Kai Wang (Ph.D. '11), who completed the work at SEAS and is now a postdoctoral
fellow at the Howard Hughes Medical Institute.

The team also found that they were able to rotate the trapped particles by
simply rotating the linear polarizer on the optical table where the experiments
were conducted. The electromagnetic field used in the experiments moved at 1014
rotations per second. However, the particle rotated at a velocity of about five
rotations per second due to fluid drag. The team calls that phenomenon a
terminal velocity.